Coagulation assays have gained acceptance as an important tool for management of patients on anticoagulation therapy for the prevention of clots within their blood vessels. In these assays, a sample of the patient's blood or plasma is tested for coagulation time or “clotting time” which time is related to the patient's dosage of the anticoagulant in the patient's blood. Coagulation assays are also required prior to surgical procedures even for patients not on anticoagulation therapy. This is because the medical professionals need to clearly know the bleeding susceptibility of the patients before they are operated on.
A variety of coagulation tests are presently in use and among the most popular are the “Prothrombin Time” (PT) test and the “Activated Partial Thromboplastin Time” (APTT) test. The PT test is based on the extrinsic coagulation pathway, and the APTT, the intrinsic coagulation pathway. Both pathways result in the formation of a proteolytic enzyme, which catalyzes the formation of the insoluble fibrin fibers from the soluble fibrinogen in the blood.
In nature, the formation of these insoluble fibers results in the formation of a mesh around a cut in the wall of a broken blood vessel, thus preventing excessive bleeding, by trapping and holding back the blood cells. Several zymogens, enzymes and coenzymes are involved in the reactions that lead to the formation of thrombin and subsequently fibrin. The absence of any of these components, which are generally referred to as coagulation factors, can impact the coagulation capability of any blood sample significantly. Each of these factors can be specifically assayed for, to determine deficiency, which is the reason for pre-surgery coagulation tests.
Blood coagulation tests have tended to be complex, and the bulk of them performed generally in centralized clinical laboratories. Clinical or a doctor's office visits on a regular basis to monitor anticoagulation therapy can be very inconvenient and expensive.
Some of the pharmacological agents used in anticoagulation therapy include heparin, protamine and Coumadin, among others. The purity and potency of these agents can vary from batch to batch, making the dosage administration difficult to the medical professional. Patients also respond differently to these agents even for the same dosage. With a very small therapeutic dose window to work with, it is imperative that the effect of these agents be monitored closely and accurately. An excess of each agent could lead to excessive bleeding and the reverse could lead to formation of clots within the blood vessels, with both cases being potentially fatal.
The prior art is replete with various apparatus and methods for measuring the coagulation time of blood samples. Most of them cannot be used for home testing while the others struggle to cope with the challenges posed by the variability exhibited by blood from patient to patient. The following, depict some examples of these devices and methods:
U.S. Pat. No. 3,695,842, which is a ‘Method and System for Analyzing a Liquid’ issued October 1972 to M.D. Mintz, and assigned to the assignee herein. The patent describes a magnetically coupled mechanical blood clot detection system wherein a variable conductance device is disposed adjacent to a zone containing a liquid and member of ferromagnetic flux lines is formed between the zone and the member. A predetermined variation in the conductance of the device is detected upon change in the magnetic flux lines when the liquid transforms itself and the member is displaced. The signal is produced at the time the predetermined variation in conductance has been detected.
An improved system of the aforementioned method for measuring clotting time is disclosed in U.S. Pat. No. 3,836,333, which is a ‘System for Timing the Coagulation of Blood’ issued to Michael D. Mintz, on Oct. 30, 1972 and assigned to International Technidyne Corporation, the assignee herein. An electromagnetic coil, which is wound around the reed switch, provides steady-state magnetic flux lines that supplement the flux lines provided by the permanent magnet. When the density of the flux lines passing through the reed switch decreases, as a result of the magnet being displaced, the reed switch opens. The bias coil also provides a magnetic pulse, which forces the reed switch to a closed state. This system is manufactured under the trademark ‘HEMOCHRON’ by International Technidyne Corporation at Edison, N.J.
U.S. Pat. No. 4,197,734 entitled ‘Apparatus for Determining Blood Clotting Time’ issued on Apr. 15, 1980 to A. Rosenberg. This patent describes an apparatus, which is capable of measuring the clotting time of blood. The apparatus includes a support frame, which supports a syringe containing a blood sample, and a turntable adapted to rotate at a normal rate of speed. Blood from the syringe drops onto the turntable where the clotting time is automatically and graphically depicted by a chart rotatively carried upon the turntable. The apparatus can also be employed to determine variations in the viscosity of blood plasma and other fluids.
U.S. Pat. No. 3,486,859 entitled ‘Blood Analyzing Method and Apparatus’ issued on Dec. 30, 1969 to R. Greiner et al. This patent depicts a blood analyzing method and apparatus including a double arm holder having blood liquid reactant chambers, which communicate with each other via a small capillary conduit. An air pump is provided for applying pressure changes to one of the chambers to effect periodic mixing of the liquids via the capillary conduit. An indicator means are included to detect the progressive restriction of the capillary conduit upon coagulation of the blood.
U.S. Pat. No. 4,797,369 entitled ‘Method and Apparatus for Detecting a Blood Clot’ issued on Jan. 10, 1989 to Michael Mintz, and assigned to the assignee herein. This patent discloses the technique for measuring clot time whereby a sample of whole blood or blood plasma is dispersed into two or more zones. The zones are separated and brought together repeatedly, such that the blood sample is divided into multiple parts, each associated with a zone. The parts are then rejoined into a single part and the process of separation and joining continues. During the process, a liquid bridge between the separated parties is initially supported by surface tension, but initially collapses at the point of maximum zonal separation. When a fibrin clot is entrained within the rejoining parts, it will align in a direction parallel to the direction of relative motion between the zones. In this manner, a thread appears between the parts as they are being separated. This thread is indicative of a clot, which clot is capable of being detected by visual or electrical means.
U.S. Pat. No. 3,890,098 entitled ‘Machine for the Determination of Prothrombin Time and P.T.T.’ issued on Jun. 17, 1975 to E. Moreno. This patent describes a reactive material, which is placed in a cup that communicates with a second cup via a restricted orifice. Plasma is placed in the second cup and the reactive material and plasma are moved from cup to cup by a pump until coagulation of the plasma takes place. Means are then provided for stopping the motion of the mixture of reactive material and plasma. Other means are provided for measuring the time required for coagulation.
U.S. Pat. No. 4,725,554 entitled ‘Method for Measuring Blood Coagulation Time’ issued on Feb. 16, 1988 to K. Schildkenecht. This patent shows a method for measuring the coagulation time of a blood sample, in which a sample reagent mixture is formed by introducing the sample and at least one reagent into a cuvette. The sample reagent mixture is moved in a stationary cuvette so that the mixture flows back and forth around an edge projecting in to the cuvette whereby a clot forms and is detected on this edge. U.S. Pat. No. 4,659,550 entitled ‘Method and Apparatus for Measuring Blood Coagulation’, describes the same system except that it utilizes photocell detectors to determine the clot formation.
U.S. Pat. No. 5,284,624 entitled ‘Method of, and Apparatus for Testing and Measuring Blood Coagulation Time’ issued on Feb. 8, 1994 to Holger Behnk. In this method, a liquid reagent and the blood sample are brought together in a cuvette, but separated by a median barrier. The cuvette and its contents are heated to the desired temperature. The cuvette is then pivoted in the measuring station by 90° resulting in a spherical stirring element falling into the reagent, and then into the sample, drawing the latter downward with it, and in that, the measurement is subsequently carried out. The measurement is based on the change in the sample's optical density. The cuvette is fed by a pump, and the spherical stirring element mixing action is driven by a magnetic stirring device. This method is used for an automatic analyzer.
Other methods used, employ multi-layered porous membranes impregnated with reagents, sometimes requiring predetermined blood volumes. The impregnated reagents initiate coagulation producing a detectable signal. Others yet employ the oscillation of magnetic particles suspended in a reagent in a changing electric field. The oscillations change as the blood sample clots. Others yet simply measure the change in light absorbance through a sample before and after the clotting reaction.
Various other systems for measuring blood coagulation and prothrombin times are found in U.S. Pat. Nos. 3,951,606; 4,659,550; and 5,302,348.
Yet another system for measuring prothrombin blood coagulation times is found in the Fibrometer™ system made by Becton, Dickinson and Company of Franklin Lakes, N.J. This system uses a stationary electrode and a moving electrode. Both electrodes are initially placed in the reaction mixture. The moving electrode then cycles up through the reaction mixture in a sweeping elliptical path. The moving electrode has a hooked end that moves in and out of the reaction mixture. When a clot forms in the sample, the clot is lifted out of the reaction mixture. When the clot forms, and is partially lifted out of the reaction mixture by the hooked end of the moving electrode, the clot closes an electrical circuit between the two electrodes such that clot formation is detected by sensing that the circuit between the two electrodes is closed while the moving electrode is in a raised position.
Most of these methods described have severe limitations which make them extremely challenging and near impossible for home use. Some require special blood preparations and handling, making them only suitable for a central clinic with well-trained staff. Some, even though possible for home use, end up being cost-ineffective for the home market. Others require sophisticated equipment with specially trained operators to run them.
The disadvantages of many of these methods, besides cost and the challenge of operation, include the fact that most do not measure coagulation directly. This has been known to pose accuracy problems in many samples. Other methods while appearing to function well, are limited to a narrow range of blood types, therapeutic windows, restricted by a long list of interfering factors and sometimes requiring large volumes of blood.
Methods that employ filtration of the sample as it percolates through its porous membranes are faced with several challenges including wetting and uniform reagent impregnation. Since some of these involve the detection of a signal that is not directly that of fibrin but some other substrate, the accuracy can be seriously compromised while the system is prone to interference from unexpected components in blood, like some medications.
Furthermore, some of these methods have components that are easily contaminated by certain blood components thereby compromising the detectable signal generated, and this could pose a real problem for the home user.
The large blood volume requirements of some of these methods make them impractical for home use. Many of these methods are also limited by what kinds of coagulation tests they can perform due to the reagent chemistry requirements and the detectable signal generated.